Natural gas is obtained from underground reservoirs and pumped through pipelines to various industrial and commercial consumers. Much of the natural gas is utilized for heating purposes and, accordingly, requires a BTU content of only 900 to 1000 BTU per m.c.f. A natural gas composed mainly of methane is sufficient to achieve such heating values. However, much of the natural gas obtained from underground reservoirs is rich in other components, such as ethane, propane, pentane and butane, which are heavier than methane. These components are industrially valuable in many processes, and, accordingly, separation of them from the methane prior to burning of the natural gas is highly desirable. Separation is usually accomplished at cryogenic temperature with distillation to separate and return methane to the gas pipe line while retaining a significant percentage of the ethane and heavier components.
Many processes have been devised for the cryogenic separation of methane from heavier components in a natural gas stream and for cryogenic refrigeration. Among these are U.S. Pat. Nos. 4,072,485 to Becdelievre et al.; 4,022,597 to Bacon; 3,929,438 to Harper; 3,808,826 to Harper et al.; Re 29,914 to Perret; Re. 30,085 to Perret; 3,418,819 to Grunberg et al.; 3,763,658 to Gaumer, Jr. et al.; 3,581,510 to Hughes; 4,140,504 to Campbell et al.; 4,157,904 to Campbell et al.; 4,171,694 to Campbell et al.; 4,278,457 to Campbell et al.; 3,932,154 to Coers et al.; 3,914,949 to Maher et al.; and 4,033,735 to Swenson.
Several of these patents, such as those to Campbell et al and Bacon, utilize a cooling mechanism by turbo expansion. Several use a plurality of successive, staged, external, indirect heat exchangers and/or totally condensing refrigerant, such as Becdelievre, Harper, Harper et al, Gaumer et al, Hughes, Coers et al, and Grunberg et al. Others use multistage flash systems separating refrigerant at various levels of temperature and pressure which emulates a cascade system in a closed loop mixed refrigerant scheme, such as those to Perret. Several also use single or very limited component refrigerant composition, such as Maher et al. Other use various schemes for refrigeration such as Swenson.
None of this prior art shows the process of the present invention for producing the cryogenic temperatures required for separation.
It is an object of the present invention to teach a method of cryogenic separation that lowers overall fuel consumption or horsepower to produce the cryogenic temperatures required.
It is a further object of the present invention to teach a method of cryogenic separation that permits broad latitude of operation. In particular, the ability to adjust the refrigerant composition to match the cooling and condensing characteristics of the feed in the process permits a degree of freedom not available in other process schemes more rigidly fixed or restrained by equipment designed for a specific process. This feature permits the process to process feed gases having a broad range of compositions, from the so-called "lean" to "rich" levels, without suffering in recovery efficiency.
It is yet another object of the present invention to teach a method of cryogenic separation wherein the turn-down capability is essentially unlimited.
It is yet a further object of the present invention to teach a method of cryogenic separation wherein the reduction in ethane and heavier hydrocarbon recovery efficiency is not nearly as pronounced on increasing through-put as it is with other systems.
It is yet an additional object of the present invention to teach a method of cryogenic separation wherein the process is less susceptible to CO.sub.2 freezing problems, accepting higher CO.sub.2 concentrations, because the demethanizer operates at warmer levels of temperature, higher demethanizer pressure, and a greater liquid-to-vapor ratio being fed to the tower.